Interleukin-18 (IL-18) was originally described in 1989 as interferon-gamma inducing factor (IGIF) and is a pro-inflammatory cytokine with various functions in addition to an ability to induce interferon gamma. These biological properties include activation of NF-κb, Fas ligand expression, the induction of both CC and CXC chemokines, and increased production of competent human immunodeficiency virus. Due to the ability of IL-18 to induce interferon gamma production in T cells and macrophages, it plays an important role in Th1-type immune responses and participates in both innate and acquired immunity. IL-18 is related to the IL-1 family in terms of both structure and function. For reviews of IL-18 structure, function and biological activity, see for example Dinarello, C. et al. (1998) J. Leukoc. Biol. 63:658-654; Dinarello, C. A. (1999) Methods 19:121-132; and Dinarello, C. A. (1999) J. Allergy Clin. Immunol. 103:11-24; (McInnes, I. B. et. al. (2000) Immunology Today 21:312-315; Nakanishi, K. et al (2001) Ann. Rev. Immunol 19:423-474.
Intracellular pro-IL-18 is proteolytically processed to an 18 kDa active form in endotoxin-stimulated cells by caspase 1 (Ghayur, T. et al., (1997) Nature 386:619-623; Gu, Y. et al., (1997) Science 275:206-209) and in Fas-L or bacterial DNA stimulated cells by caspases 4, 5 and 6 (Tsutsui, H. et al., (1999) Immunity 11:359-67; Ghayur, T., Unpublished Observations). Pro-IL-18 is also proteolytically processed by other proteases such as neutrophil proteinase 3 (Sugawara, S. et al., (2001) J. Immunol., 167, 6568-6575), caspase 3 (Akita, K. et al., (1997) J. Biol. Chem. 272, 26595-26603), and serine proteases elastase and cathepsin (Gracie J. A., et al., (2003) Journal of Leukocyte Biology 73, 213-224). Both human and murine IL-18 lack a classical leader sequence and the mechanism of mature IL-18 release by cells is not well understood.
The biological activities of IL-18 are mediated through IL-18 binding to a heterodimeric IL-18 receptor (IL-18R) that consists of two subunits: the α-subunit (a member of the IL-1R family, also termed IL-1R-related protein-1 or IL-1Rrp1) and the β-subunit (also termed IL-18R accessory protein, IL-18AP or AcPL). The IL-18Rα subunit binds IL-18 directly, but is incapable of signal transduction. The β-subunit does not bind IL-18 by itself, but in conjunction with the α-subunit forms the high affinity receptor (KD=˜0.3 nM) that is required for signal transduction (Sims, J. E., (2002) Current Opin Immunol. 14:117-122). IL-18 signal transduction via the IL-18αβ complex is similar to the IL-1R and Toll like receptor (TLR) systems. IL-18R signaling uses the signal transduction molecules, such as MyD88, IRAK, TRAF6 and results in similar responses (e.g. activation of NIK, IkB kinases, NF-kB, INK and p38 MAP kinase) as does IL-1. Requirement for IL-18Rα and signal transduction molecules in mediating IL-18 bioactivity has been confirmed using IL-18Rα subunit (Hoshino K., et al (1999) J. Immunol. 162:5041-5044;), MyD88 (Adachi O., et al. (1998) Immunity 9:143-150) or IRAK (Kanakaraj P., (1999) J. Exp. Med. 189:1129-1138) knockouts respectively.
Antibodies that bind IL-18 are known in the art. Mouse antibodies capable of neutralizing IL-18 are disclosed in EP 0 974 600. Human antibodies to IL-18 have been disclosed in PCT publication WO 0158956 and incorporated herein by reference. The present invention provides a novel family of binding proteins, human antibodies, and fragments thereof, capable binding IL-18, binding with high affinity, and binding and neutralizing IL-18.